This invention generally relates to hermetically-sealed electronic devices and, more particularly, to compositions and methods for removing hydrogen and moisture from such devices.
Electronic assemblies for use in a variety of harsh environments, such as in space or marine applications, oftentimes are sealed from the outside conditions by use of gas-tight (hermetically sealed) containers. Such containers, while sealing out external dust, air and the like, trap in whatever is inside the devices.
Such hermetically sealed electronic assemblies generally contain semiconductor devices made from gallium arsenide (chemical symbol: GaAs) and/or indium phosphide (InP), and, in order to assure reliability and minimize failures in use, are test-operated at elevated temperatures (typically in the range of 125.degree. C.) for a period of time (typically 1,000 hours), generally referred to as burn-in and life testing. At such conditions, a loss of performance was found both in alternating current (AC), including radio frequency (RF), operating characteristics, as well as direct current (DC) operating characteristics. It was determined that hydrogen gas was the primary cause of the performance degrading. This degradation was later observed to be common to most GaAs semiconductor devices utilizing the industry standard gate metallization structures. (Structures made with titanium/platinum/gold (Ti/Pt/Au) or titanium/palladium/gold (Ti/Pd/Au) gates)(Kayali, "Hydrogen Effects on GaAs Device Reliability," 1996). The source of the problem has been determined to be hydrogen gas that was absorbed in the package metals (Kovar, etc.) or hydrogen generated by other materials such as RF absorbers inside the device package. Hydrogen gas desorbed or generated inside hermetically sealed devices has no clear path for escape, and its concentration can easily rise to 1-2% of the gas volume in gas-filled devices during initial operation.
The exact mechanism by which hydrogen degrades device performance, and the path by which hydrogen reaches the active area of a device is not well understood and is still a subject of investigation. However, it appears that the platinum group metals Pt and Pd used in gate structures play an important role in the degradation process (Camp et. al., "Hydrogen Effects on Reliability of GaAs MMICs," GaAs IC Symposium, 1989). These metals are catalysts for the dissociation of molecular hydrogen to atomic hydrogen which can diffuse into other areas of the device. Earlier research on GaAs transistors identified the diffusion of atomic hydrogen directly into the channel area of the device where it neutralizes the silicon donors as a possible mechanism (Chevallier et al., "Donor Neutralization in GaAs(Si) by Atomic Hydrogen," Appl. Phys. Lett. 47, pg 108, July 1985.) Whatever the precise mechanism of device degradation by hydrogen may be, it is clear that it has a direct impact on the performance and reliability of GaAs devices used for high reliability applications.
One typical means of addressing this problem is the use of compositions which scavenge the hydrogen, generally referred to in the art and herein as "hydrogen getters" or, simply, "getters." Microcircuit devices, and, thus, the getters may be required to function in a vacuum, or be air or inert gas filled devices. Furthermore, they typically are required to function at temperatures ranging from -55.degree. C. to 150.degree. C., or higher. Space inside such devices is limited and, thus, it is highly preferable that the getter may be easily formed into a thin film or similar shape to conform to the inside of the sealed device. Further, to be a viable remedy, the getter should lower hydrogen concentrations to the low parts per million level (100 PPM or lower is preferable) over the operating temperature range of -55.degree. to 150.degree. C. where GaAs or similar devices are commonly used. It also must not desorb any other materials that might degrade device performance. Ideally, it should be easily manufactured to any desired physical dimensions and have a hydrogen capacity which could be tailored for any particular application.
One type of prior art hydrogen getter consists of alloys of metals such as iron, nickel, titanium, vanadium, zirconium, chromium, cobalt, the rare earth metals, and other metals and alloys which react with hydrogen to form metallic hydrides. These metallic getters typically require high temperatures exceeding 300.degree. C. for activation and/or operation, and are frequently poisoned by the presence of oxygen, water vapor, or other contaminants such as chlorine gas and the like. Thus, this type of hydrogen getter may be undesirable where power or temperature constraints limit the hydrogen getter temperature or such contaminants are present. As with other metals, these alloys can be metal-worked to form thin films for insertion into sealed devices, although the metal-working machinery to perform such stamping may be unwieldy and shaping the alloys into complex forms may be difficult.
Another type of prior art hydrogen getter consists of a mixture of a platinum group metal, usually palladium for cost reasons, on a substrate, such as activated carbon, to increase active surface area and an unsaturated organic compound, such as diphenyl butadiyne or 1,4-bis(phenylethynyl) benzene. The platinum metal operates as a catalyst in the mixture to bind the hydrogen into the organics. This type of getter will function at low to moderate temperatures (less than 100.degree. C.) and is not poisoned by oxygen or water vapor. However, these materials have significantly high vapor pressures at temperatures above 100.degree. C. and may melt in that range, which may lead to material migration inside the device, which is very undesirable. Although this type of getter is not poisoned by the presence of water vapor, neither does it have any capabilities for absorption of water vapor. Any water vapor present or produced inside of an electronic enclosure is a potential source of corrosion, and circuit shorting and thus is undesirable. Typically, this type of getter is manufactured from a commercially-available palladium-on-activated-carbon powder which is mixed with the desired organic material in ajar mill or the like to form a finely-ground and well-mixed powder. The powder may then be molded into a pellet shape in a pellet maker. However, the resulting mixture is very brittle and, thus, is not suitable for molding into thin shapes.